"Such a beautiful evening and in an hour, they say, England will be at war," wrote WH Auden on 1 September 1939 in an unpublished diary that sheds light on the composition of one of his most famous poems.

The journal was one of just three kept by the British poet. It had been in private hands since Auden's death in 1973, but was recently unearthed and sold earlier this month at Christie's in London to the British Library for £47,475. Christie's called it "the most substantial and significant Auden manuscript to have been offered at auction", and said it offered "an incomparable insight into the poet's activities and reflections at the turning point in his life".

Auden wrote the journal between August and November 1939, shortly after he left England for America with the novelist Christopher Isherwood – a move heavily criticised as unpatriotic by the British media. But Auden's journal shows that, despite his absence, events in Europe were very much on his mind, said Helen Melody, curator of modern literary manuscripts at the British Library.

Auden's famous poem, September 1, 1939, opens "I sit in one of the dives / On Fifty-second Street / Uncertain and afraid / As the clever hopes expire / Of a low dishonest decade". On the same day in 1939 the poet wrote in his diary of how he "woke with a headache after a night of bad dreams in which C [Chester Kallman, the American poet] was unfaithful. Paper reports German attack on Poland … 6.0pm."

He tells of how Benjamin Britten and Peter Pears came to lunch, how "Peter sang B's new settings of Les Illuminations and some H Wolf … which made me cry. B played some of Tristan which seems particularly apposite today. Now I sit looking out over the river. Such a beautiful evening and in an hour, they say, England will be at war … 10.30 Went to the Dizzy Club. A whiff of the old sad life. I want. I want. Je ne m'occupe plus de cela. Stopped to listen to the news coming out of an expensive limousine."

War declared this morning… a page from the journal. Photograph: Christie's/PA

Two days later, on 3 September, Auden writes: "war declared this morning at 7am. Listened in the afternoon to a broadcast of the first 1½ acts of Tristan. Everyone very kind, some rather drunk. The frogs sang all night. We sang spirituals out on the lawn."

"Although he was not in Europe he was very much aware of what was going on," said Melody. "At this point in the journal he is thinking about the wider opinion of himself and his actions … in a little note from November, he's reflecting upon 'What people in England will think of [JB] Priestley's remark about deserters'."

The journal opens as Auden returns to New York from California in August 1939, having spent "the eleven happiest weeks of my life" with Kallman. The couple met at a public poetry reading, and their relationship was "instrumental", the British Library said, in Auden's decision to become an American citizen.

"At 32½ I suppose I shall not change physically very much for some time except in weight which is now 154lbs … I am happy, but in debt … I have no job. My visa is out of order. There may be a war. But I have an epithalamion to write and cannot worry much," the poet writes.

The journal records Auden's thoughts on topics from women ("My hatred of women is such that if I am not afraid of them … I am cruel") to politics. "The problem for a democracy is how to get rid of the pitiful vanities of partisan talk and voting and the corruption of party machines without silencing opposition criticism," he muses.

It also provides a rich supply of what Christie's called "typically idiosyncratic aphorisms and observations". "All the great heretics, Pascal, Rousseau, Lawrence, Kafka etc have been sick men," Auden writes, going on to observe that "all bureaucrats should be obliged to prove that they have a happy love-life, and immigration officials most of all", and that "it is impossible to listen to music and get an erection at the same time".

He also criticises "the American habit of washing one's hands after pissing, as if the penis were an object, too filthy for any decent person to touch".

The British Library will put the manuscript on display from August. "It's been in private hands for a number of years and this will be the first time it is widely available," said Melody.